build: don't define _GNU_SOURCE manually

Now that we're using gnulib in earnest, any manual definition
would provoke a redefinition warning.
* fish/fish.c (_GNU_SOURCE): Don't define.
* fish/destpaths.c (_GNU_SOURCE): Likewise.
* src/guestfs.c (_GNU_SOURCE): Likewise.
* bootstrap (modules): Add asprintf, strchrnul, strerror, strndup
and vasprintf.
* fish/fish.c (main): Set argv[0] to sanitized program_name, so
functions like getopt_long that use argv[0] use the clean name.

bootstrap

@@ -50,6 +50,7 @@ gnulib_tool=$GNULIB_SRCDIR/gnulib-tool
 (cd daemon && mkdir -p tests lib && ../$gnulib_tool --update)
 
 modules='
+asprintf
 closeout
 gitlog-to-changelog
 gnu-make
@@ -58,8 +59,12 @@ ignore-value
 maintainer-makefile
 manywarnings
 progname
-warnings
+strchrnul
+strerror
+strndup
+vasprintf
 vc-list-files
+warnings
 '
 
 $gnulib_tool			\

fish/destpaths.c

@@ -18,8 +18,6 @@
 
 #include <config.h>
 
-#define _GNU_SOURCE		// for strndup, asprintf
-
 #include <stdio.h>
 #include <stdlib.h>
 #include <stddef.h>

fish/fish.c

@@ -18,8 +18,6 @@
 
 #include <config.h>
 
-#define _GNU_SOURCE // for strchrnul
-
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>

src/guestfs.c

@@ -19,7 +19,6 @@
 #include <config.h>
 
 #define _BSD_SOURCE /* for mkdtemp, usleep */
-#define _GNU_SOURCE /* for vasprintf, GNU strerror_r, strchrnul */
 
 #include <stdio.h>
 #include <stdlib.h>